This invention relates to withdrawal of liquid and gaseous products of retorting from an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597 and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary, fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable that oxygen-supplying gas flow relatively uniformly through a fragmented mass during retorting operations. Gas channeling through the fragmented mass can occur if there is non-uniform permeability in horizontal planes across the fragmented mass. By producing a generally uniform flow of gas across the horizontal cross-section of the fragmented mass during retorting, bypassing portions of the fragmented mass by retorting gas can be reasonably avoided, producing a reasonably uniform conversion of kerogen to liquid and gaseous products during retorting. Such a uniform conversion of kerogen to liquid and gaseous products can increase product yield from the fragmented mass when compared with a retort in which gas flow is not reasonably uniform.
Gaseous products of retorting in an in situ oil shale retort can include a process off gas that contains nitrogen, hydrogen, carbon monoxide, carbon dioxide, water, vapor, methane, and other hydrocarbons and sulfur compounds, such as hydrogen sulfide. Hydrogen sulfide and carbon monoxide are extremely toxic gases. For this reason, such off gases have been withdrawn from the fragmented mass through a gas withdrawal drift that is sealed against the passage of off gas from the portion of the drift where the off gas collects, so that workers in adjacent underground workings are isolated from the off gas. Such a gas withdrawal drift is commonly sealed with a bulkhead placed across the drift. Such a bulkhead can comprise a steel bulkhead plate secured to a rigid framework, with concrete anchoring the peripheral portion of the bulkhead to provide a substantially gas-tight seal across the drift.
In other methods for withdrawing liquid and gaseous products of retorting from a fragmented mass, a substantial constriction can be created in the horizontal cross-sectional area through which gas can flow between the upper regions of the fragmented mass and a production level drift. Such a constriction to gas flow can increase gas velocities in the lower portion of the fragmented mass to as high as five to ten times the velocity of gas flow in the upper elevations of the fragmented mass. Such a high velocity can entrain fine droplets of shale oil in the gas flowing through the lower portion of the fragmented mass, producing aerosols which are withdrawn in the retort stack gas. To maximize the product yield of the retort, it is desirable to minimize the amount of shale oil withdrawn as an aerosol in the retort stack gas.
The present invention provides techniques for withdrawing liquid and gaseous products of retorting from a lower portion of a fragmented mass such that the need for a conventional gas-tight bulkhead sealed across a production level drift can be eliminated. The invention thus can reduce the time and cost required in forming a gas seal in a production level drift because of materials cost and labor for constructing a conventional bulkhead structure can be avoided. The present invention also avoids the chance of gas leakage through such a bulkhead, and gas leakage through formation around such a bulkhead also is alleviated.
The present invention also can minimize the amount of shale oil withdrawn as an aerosol in the retort stack gas, as well as providing means for reasonably controlling the uniformity of the gas flow profile through the fragmented mass during retorting.
Some in situ oil shale retorts have a gas outlet at one side of the bottom of the fragmented mass and non-uniform gas flow can be encountered. It is desirable to have distributed gas outlet means at the bottom of an in situ retort for promoting uniform gas flow.